Can it be equipped with 18/21 but both tubeless? A puncture would take 5min to fix in the middle of nowhere. Isn't this the goal for an adv bike? Why go with tubes and have to carry heavy tools for a simple puncture that would take hours to fix on your own in the wild...?

There are one or two 21" TL tires, but no mfg seems to want to try running them stock and there are of reports of debeading issues after deflation by those who have tried them with sealed rims - debeading of a TL tire is extremely dangerous. Whether that's a tire problem, rim problem, or something inherent in the larger tire that makes TL an issue, I don't know.

The tube vs. TL debate is an old one. I'm in the TL camp, but for hard off-roading, there are some arguments to be made for tubes which provide for better repair-ability of unusual punctures. For an big adventure bike like this, I think 17"-19" and TL is the best compromise, but if Yamaha can sort a 21" TL, that would be a nice differentiator from the big GS.